1. Field of the Invention
The present invention relates to an optical demultiplexing system which separates optical signals having different wavelengths and transfers the separated optical signals to predetermined receivers on the receiving side in an optical fiber transmission system or an optical signal processing system that uses optical wavelength multiplexed signals.
2. Description of the Related Art
The recent development of a large-capacity optical fiber transmission system points toward the use of a wavelength (frequency) multiplexing system as well as a time-sharing multiplexing system. FIG. 3 shows a signal distributing system employing an optical wavelength multiplexing system for distributing signals to a subscriber loop system. An office, i.e., a central office or a remote terminal, comprises a M-channel optical transmitter 4 comprising DFB lasers which emit laser light having different wavelengths, respectively, and an M.times.N star coupler 5 (M=N=16) not having wavelength dependency. In each channel, signals of 600 Mbit/s or 2 Gbit/s are multiplexed by the direct modulation of the lasers, the star coupler 5 multiplexes sixteen wavelengths and branches the optical power into sixteen to send out an optical multiplexed signal to the subscriber. The operating wavelengths of the sixteen DFB lasers are in the range of 1527 to 1557 nm and the spaces between the operating wavelengths are 2 nm. The optical wavelength multiplexed signals travel 10 km and reach the subscribers. A 1.times.4 optical coupler 7 on the subscriber's side receives the optical wavelength multiplexed signals and distributes the same to four ports, an etalon-type variable-wavelength filter 8 connected to the output of each port extracts an optical signal of a desired wavelength from the 16 optical wavelength multiplexed signals, and then an optical receiver 9 converts the extracted optical signal into a corresponding electrical signal and processes the same. According to the selected purpose either only one of the four channels or all the four channels of the optical coupler 7 may be used. The variable wavelength filter 8 has a free spectrum range of 52 nm, and a band width of 0.25 nm. This system is capable of transmitting signals at 2 Gbit/s for each channel at a bit error rate of 10.sup.-9 for sixteen channels. Accordingly, the subscriber is free to select information of a large quantity of 32 Gbit/s.
Techniques relating to this system are described in, for example, Electronics Letters, vol. 24, No. 19, pp. 1215-1217 (1988).
The above-mentioned known system employs passive devices including optical couplers and optical filters to multiplex, distribute and demultiplex optical wavelength multiplexed signals, and the passive devices cause optical insertion losses. In the system shown in FIG. 3, an optical insertion loss caused by each channel is 14 dB in an average, an optical loss caused by the 1.times.4 coupler 7 is 8.5 dB, an optical loss caused by the variable wavelength filter 8 is 6 dB, and hence the total optical insertion loss is 28.5 dB, which is about seven times the transmission loss of 4 dB caused by the optical fiber. Thus, the insertion losses caused by the passive devices can be the limiting factor in terms of the transmission distance. Accordingly, in some cases, wavelength multiplexing transmission is infeasible if the number of the passive optical devices of the subscriber's side is increased to increase the degree of wavelength multiplexing, the insertion loss attributable to each passive optical device increases or when the transmission distance of the trunk or the LAN exceeds 10 km.
FIG. 4 shows the basic configuration of a trunk transmission system employing an optical wavelength multiplexing. A transmitting side comprises a plurality of optical transmitters 10 which generate optical signals respectively having different wavelengths, and an optical wavelength multiplexer 11 which multiplexes the optical signals provided by the optical transmitters 10 to provide an optical wavelength multiplexed signal and applies the signal to a single optical fiber 12. Each optical transmitter 10 is provided, for example, with a laser diode. A receiving side comprises an optical demultiplexer 13 which separates the component optical signals respectively having different wavelengths, and a plurality of optical receivers 14 respectively for converting the separated optical signals into corresponding electrical signals. Each optical receiver is provided, for example, with a photodiode. Consider the 2 Gbit/s 16-channel system previously described in connection with the aforesaid known signal distributing system and suppose that the optical output of the optical transmitter 4 is 0 dBm and the sensitivity of the optical receiver 9 is -30 dBm. If the optical wavelength multiplexer 11 and the optical demultiplexer 13 are provided each with a 1.times.16 star coupler causing an insertion loss of 14 dB, and the wavelength selector of the optical demultiplexer 13 is provided with a wavelength filter causing an insertion loss of 6 dB, the total insertion loss of the transmission system is 34 dB, the optical power arriving at the optical receiver 14 is -34 dBm lower than the receiver sensitivity and, consequently, the optical signals cannot be transmitted to the receiving side.
FIG. 5 shows another transmission system proposed to solve such a problem (Denshi Joho Tsushin Gakkai Shuki Zenkoku Taikai 1990, B-780, pp. 4-115). This transmission system includes an optical amplifier. Referring to FIG. 5, a transmitting side comprises an optical transmitter 15 comprising four laser diodes of 10 Gbit/s each for one channel, capable of generating optical signals respectively having different wavelengths, and a 1.times.4 optical coupler 16. An optical signal produced by 4-wave multiplexing is transmitted through a single optical fiber 17 of 40 km in length to an optical filter 18. The optical filter 18 extracts a component optical signal of a desired wavelength from the received optical wavelength multiplexed signals, an optical amplifier 19 amplifies the extracted optical signal, an optical filter 20 removes noise related to the amplified spontaneous emission in the optical amplifer 19, and then an optical receiver 21 converts the output signal of the optical filter 20 into a corresponding electrical signal. The optical amplifier 19 makes compensates for the losses caused by the optical coupler 16, the optical fiber 17 and the optical filter 18 to amplify the optical signal to a level corresponding to the sensitivity of the optical receiver 21. Thus, the use of an optical amplifier in a system provided with passive devices that cause large insertion losses is effective. However, if the degree of wavelength multiplexing is increased, the optical demultiplexer disposed above the optical receiver will need a plurality of optical amplifiers. No study has been made concerning the arrangement of a plurality of optical amplifiers in the optical demultiplexing system. Thoughtless arrangement of optical amplifiers will increase the number of optical amplifiers excessively inevitably increasing power consumption. Since the power consumption of one current optical amplifier is 0.5 W or above, ten optical amplifiers will consume power of 5 W or above.